Hyde here from Shopify. Very interesting question!
Starting and running an online business is much the same as running a physical store. The more time and care you put into it, the better your chances of success. As a consequence, we recommend picking one store, one product line, and focusing your attention on just one account.
Whether you want to sell your own product or try dropshipping is entirely your prerogative. It's worthwhile noting that either way, you will need to find a niche to distinguish your brand from your competitors. It will have to be a niche that you are knowledgeable in and that you can establish yourself as an authority in.
Here is a really great article outlining the pros and cons of every e-commerce business model. I'd recommend reading through that to get a sense of which business model you prefer, and then focusing all your effort on that. If you are going to dropship, you might want to consider an app like Oberlo (here's how it works). Alternatively, you might be interested in Print on Demand, which is also a pretty great business model. You can get a better sense of how that works by reading this blog post.
I hope that helps, please let me know if you have any further questions.
All the best, Hyde.